The present invention relates generally to thermodynamic machines and processes for producing heating, refrigeration, or work from thermal energy. More particularly, the invention relates to a piston machine operable as a heat pump, a refrigerator, or a heat engine.
The transformation of heat into work is typically accomplished by two general types of engine: the external heat source engine, such as the steam engine, and the internal heat source engine, such as the gasoline engine and the diesel engine. In both types of engines, a working medium, such as a vapor or a gas or a mixture of gases contained in a cylinder, undergoes a cycle, thereby causing a piston to impart to a shaft a motion of rotation against an opposing force. It is necessary in both types of engines that, at some time in the cycle, the vapor or gas in the cylinder be raised to a high temperature and pressure. In the external heat source engine this is accomplished by an outside heat source, such as a boiler. External heat source engines are of two general types; condensing cycle engines, such as the steam engine, and non-condensing cycle engines.
One of the earliest developed non-condensing, external heat source engines is the Stirling engine, a hot-air engine that converts some of the energy liberated by burning fuel into work. An ideal Stirling engine runs on the Stirling cycle, which is characterized by constant temperature (isothermal) expansion and compression and constant volume (isochoric) heating and cooling. A typical Stirling engine includes two pistons, an expansion piston and a compression piston, connected to the same shaft. As the shaft rotates, the pistons move in different phase, with the aid of suitable connecting linkages. The space between the two pistons is filled with a working medium, typically air, helium, or hydrogen. The expansion piston is kept in contact with a hot reservoir (i.e., burning fuel), while the compression piston is in contact with a cold reservoir. A regenerator is positioned between the two pistons. The regenerator is constructed to have a thermal conductivity low enough to support the temperature difference between the hot and cold reservoirs without appreciable heat conduction.
Recently, there has been renewed interest in developing Stirling cycle engines. This is due to the high theoretical thermal efficiency that can be attained with a Stirling cycle engine. With perfect (100%) regeneration, the ideal Stirling cycle engine has Carnot cycle efficiency (1-T.sub.c /T.sub.h). Stirling cycle engines have been constructed using ceramic materials on the hot side of the engine to allow the engine to achieve over 40% thermal efficiency. Furthermore, the external combustion process allows the use of a wide variety of combustible or renewable (i.e., solar) fuels. Also, Stirling cycle engines are generally quieter and produce lower emissions than conventional internal combustion engines.
A further advantage of the Stirling cycle engine is that the engine is a reversible device. This allows the Stirling engine to be operated in a refrigeration or heat pump cycle in which net work is done on the engine and heat is accepted at a low temperature and rejected at a high temperature. When the engine is used as a refrigeration or cooling device, the useful heat is the heat accepted at the low temperature. Conversely, when the engine is used as a heating device the useful heat is the heat that is rejected at the higher temperature.
In a conventional refrigerator or heat pump, a compressor delivers gas, known as the refrigerant, at a high temperature and pressure to condensing coils. Heat is removed from the gas by cooling, resulting in condensation of the gas to a liquid still under high pressure. The liquid passes through a throttling or expansion valve, emerging as a mixture of liquid and vapor at a much lower temperature. Heat is supplied to the gas in an evaporation coil that converts the remaining liquid into vapor which enters the compressor to repeat the cycle. One of the problems associated with conventional refrigerators is the need for a refrigerant that releases a large amount of latent heat in relation to its volume when undergoing a change of state. Such refrigerants can be expensive to manufacture and can be harmful to the environment.
As a heating/cooling device, the Stirling engine offers advantages over devices that operate according to a condensing cycle, such as the conventional refrigerator. By avoiding changes of state and inevitable thermodynamic losses due to the throttling process in conventional refrigeration or heat pump cycles, and by incorporating a regenerator that recycles heat energy that would otherwise be lost, a Stirling cycle offers significant theoretical advantages over those systems in energy consumption, and superior coefficients of performance in operation. Additionally, the Stirling cycle does not necessitate the use of conventional refrigerants.
One of the limitations associated with Stirling cycle engines is that the engine is practically limited to equivalent compression and expansion ratios of 2.5 or less and thus requires high pressurization of the working medium to attain sufficient mass flow for practical work output, greatly complicating engine mechanical design. The Stirling engine also relies heavily upon the ability of the regenerator to store heat during the constant volume phases of the Stirling cycle in order to reach maximum thermal efficiency. Such high efficiency regenerators can be difficult and expensive to manufacture and introduce additional flow losses which reduce the practically achievable efficiency to a fraction of theoretical values.
Accordingly, there is a need for an external heat source, non-condensing thermodynamic system having a thermal efficiency approaching the theoretical thermal efficiency of the Stirling engine while concomitantly avoiding the high pressurization required by low compression/expansion ratios and the frictional flow losses through the Stirling regenerator.
An object of the present invention is to provide an improved thermodynamic system that is capable of operating as a heat engine, a refrigerator, or a heat pump with high thermal efficiency.
Another object of the present invention is to provide a thermodynamic system that can operate as a thermally efficient heating or cooling device without the need for expensive or environmentally harmful refrigerants.
Still another object of the present invention is to provide a thermodynamic system that can operate as a thermally efficient heating or cooling device by incorporating a cyclic recuperator with minimal flow losses and high heat recycling efficiency.
Other general and more specific objects of this invention will in part be obvious and will in part be evident from the drawings and the description which follow.